We are currently screening a library of mouse genomic DNA cloned in the bacteriophage Charon 4A to identify sequences which are specifically expressed in mouse colon tumor (transplantable and induced by dimetylhydrazine) and not in the normal colon. The 32P probes of normal and tumor RNA have been prepared by chromatography of the RNA on oligo dT cellulose and reverse transcription of the poly A ion fractions. They are used for screening by the plaque-filter technique of Benton and Davis (Science 196, 180, 1977). To confirm that isolated sequences are relatively abundant in the tumor, but not in normal tissue, 32P end labelled poly A ion RNA from normal and tumor will be hybridized to the isolated cloned DNA bound to filters. Quantitative in situ hybridization of high specific activity cloned probes will also be used to investigate the specificity of the sequences and to follow their expression during chemical induction of colon tumors.